PWM (Pulse Width Modulation) techniques are frequently used in power electronics to drive large load currents because of their high efficiency. In comparison, linear currents are almost never used in driving large load currents because of their poor efficiency. However, because there are no switching signals in a linear current source, the output current will not have any ripple. A PWM driven load current will inevitably experience some ripple, the amplitude dependent on the cutoff frequency and attenuation of the filtering network.
Four power MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) connected in an H-bridge are commonly used to drive a differential load. FIGS. 1 and 2 depict conventional H-Bridge circuits 10 used to drive a load. The H-Bridge circuit depicted in the figures includes four switches (12, 14, 16 and 18) arranged as shown to drive a load 19, as is well understood in the art.
FIG. 1 depicts current flowing from left to right, and shall be defined herein as “cooling”. FIG. 2 depicts current flowing from right to left and shall be defined herein as “heating”. FIGS. 1 and 2 show the signals required to drive 4 H-bridge connected power MOSFETs to drive a resistive load in the heating and cooling mode. For example, to drive the load in the cooling mode (FIG. 1), PWM signals are applied to P1 and N1. P2 is disabled and N1 is fully turned on. This operation is similar to a buck converter, as is known in the art. The duty cycle of the PWM signal will control the current flowing to the resistive load. Filter elements L1, C1, L2 and C2 will attenuate the ripple current though the load. Filter elements L1, C1, L2 and C2 will attenuate the ripple current through the load. Each switch has an associated pre-driver circuit (not shown) that drivers the switch at an appropriate level.
Such a design will experience some problems when small current is required through the load. At small load current, the duty cycle of the PWM signals is correspondingly reduced. However, the driving capability of pre-divers circuits is limited in terms of duty cycle. Moreover, the gate capacitance of power MOFSETs are quite significant. Hence, it is not possible to drive the MOFSETs at very small duty cycle. Moreover, the gate capacitances of the power MOFSETs are quite significant. Hence, it is not possible to drive the power MOFSETs at very small duty cycle resulting in the system not being able to output small load current in either the heating or cooling mode. Likewise, the percentage of ripple current will increase significantly as the average DC value of the load current decreases.
FIGS. 3 and 4 show another variation of FIGS. 1 and 2 in which one set of the filter elements, L2 and C2, is removed. This generally results in costs saving and a smaller form factor. However, the circuit 20 of FIGS. 3 and 4 still suffers from the same deficiencies as the circuit of FIG. 1 and 2, i.e., increased ripple at low current conditions.